1. Field of the Invention
The present invention relates to certain improvements in the invention forming the subject matter of my copending application above-identified in which I not only am able to generate short optical pulses brought about by the interference between frequency-shifted waves of different frequencies circulating in an active cavity and fed back into the gain medium during each pass therethrough but, in addition, I am now able to generate a band of light having a broad spectral output. A continuous wave laser is constructed in which the light is shifted in frequency before being fed back into the gain medium. An acousto-optic modulator, an acoustic grating or other type of conventional frequency-shifting device is used inside the cavity to incrementally shift the frequency of the light circulating therein during each pass. A system of one or more mirrors are used within the cavity to circulate the light waves through the frequency shifter and the gain medium with the length of the cavity being selected to have such waves arrive in phase with those preceding it in those instance where a pulsed output is desired.
2. Description of the Related Art
The closest prior art known to me is contained in U.S. Pat. Nos. 3,834,790 to Macken and 4,586,184 to Hess, both of which are of record in my parent application previously identified along with other less pertinent art. An essential feature of the active cavity used in accordance with the teaching of the present invention is that it include a frequency-shifting device, a gain medium and a system of mirrors by means of which the waves of light will circulate therein and have the frequency thereof incrementally increasing or decreasing during each pass without any modes being produced.
Of the two, Macken is the least pertinent of the two. The teaching of the Macken patent is strictly one of coupling a beam of light having a single known frequency into a passive cavity and shifting its frequency therein to one which more nearly matches the reflected beam returning from a moving target such that the beat frequency resulting from the comparison of the two in accordance with the well known Doppler effect will fall within the range of the detection apparatus. Light of a single frequency, shifted or not, is incapable of generating pulses or a continuous wave of broad spectral content; therefore, Macken has nothing to say of any significance to the teaching found herein.
The Hess patent is a complete enigma because my investigation has conclusively established that the set up he illustrates in FIG. 4, for example will not generate a chirped output as he claims. His various embodiments including the one shown in FIG. 4 which most nearly corresponds to mine and includes an active medium in the form of a dye jet pumped with a Krypton laser will not produce a mode structure in his active cavity and the absence of such a mode structure renders the output of a chirped signal impossible. Since my findings are to the effect that such a system is modeless and results in a continuous wave output of broad spectral content, I can only conclude that the systems shown on the Hess patent were merely matters of conjecture and not actually built and tested to prove or disprove his hypothesis. I admit that one experienced in the laser art might expect that a standing wave would exist in such a cavity but, contrary to such an expectation, and to my surprise, none was found. Instead there were no modes in the cavity, the output had a broadband spectral content and at least at the present time I suspect, but have not yet proven, that such a broadband output is pulsed or continuous.
The standard, and insofar as I am aware, the only method for sustaining laser oscillation is through the use of the feedback in a Fabry-Perot cavity. The multiple reflections within this type of cavity lead to destructive interference for most frequencies, however, for the discrete frequencies which correspond to the standing waves of the cavity, the interference is constructive and the intensity becomes quite large.
The dominant characteristic of this type of laser is that the output power is distributed in a narrow spectral region, the modes of the cavity. In my system, on the other hand, a modeless continuous or pulsed wave laser is constructed in which the spontaneously emitted light from the dye jet of a dye laser provides the broadband input to the frequency-shifted feedback cavity instead of light of one or more discrete frequencies coupled into a passive cavity from an external source. When this is done, there is no Fabry-Perot frequency discrimination because the interference from multiple reflections which takes place is between waves of different frequencies and, as a result, there is no mode structure as the prior art according to Hess would incorrectly suggest. The broadband spectrum of the spontaneously-emitted light is fed back into the gain medium with no frequency discrimination apart from the dispersive effects of the acoustic grating or other diffraction medium. Accordingly, an optical spectrum analysis of the light circulating within the cavity will show the same pattern for each frequency circulating therein which fact shows that the frequency-shifted feedback cavity does not selectively attenuate input frequencies like a Fabry-Perot interferometer does. Once again, the prior art teaching found in Hess suggests that such a frequency-shifted feedback cavity will filter and thus selectively attenuate the input frequencies in the same manner as the Fabry-Perot interferometer, however, I find that this is not the case and the laser will, in fact, oscillate with a broad spectral output especially when the output is continuous and it may well even do so when it is pulsed. I can generate pulses of varying rate as well as enhance overall pulse production by incorporating within the cavity along with an active medium some type of frequency selective element or elements.